This invention relates to an apparatus that encodes video signals efficiently with fewer codes and an apparatus that decodes the coded signals, in their use for recording, transmission and display systems, and more particularly to a coding/decoding apparatus for video signals capable of random access and search.
In particular, efficient coding has been realized by interframe predictive coding which utilizes interframe correlation of video signals to predict, from already coded frames, the frames to be coded, and encodes predictive error signals alone.
Furthermore, recent attention has been directed to the technology of motion-compensating interframe prediction to predict motion by moving pictures.
On the other hand, it has been made for the coding for storage media to respond to random access and high-speed search.
A conventional coding apparatus is exemplified in FIG. 1.
In FIG. 1, a video signal applied to a video signal input terminal 1 is led to a predictive subtracter 3, where a predictive signal given via a switch 15 is subtracted from the video signal, thus generating a predictive error signal.
The predictive error signal is encoded by an intraframe encoder 5 into compressed data. The data is outputted via a data output terminal 7 and applied to an intraframe decoder 9 which generates a reproduced predictive error signal.
In an inverse predictive adder 11, a predictive signal is added to the reproduced predictive error signal, thus reproducing the video signal.
The reproduced video signal is stored in a frame memory (FM) 13, and applied to a terminal P of the switch 15 after being delayed by one frame.
The switch 15 is controlled by a synchronous signal separated from the inputted video signal. And, in an independent frame of every N frames, a terminal S is connected to a terminal I, whereas in other predictive frames the terminal S is connected to terminal P.
The terminal P has been supplied with a reproduced video signal of the preceding frame. An interframe predictive coding circuit is formed when the terminal S is connected to the terminal P.
The terminal I has been supplied with a fixed value (0). When the terminal S is connected to the terminal I, the output signal of the predictive subtracter 3 becomes the same as the input video signal, that is, an intraframe independent coding circuit is formed.
FIG. 2 shows a conventional video signal decoding apparatus. In this apparatus, the data supplied from a data input terminal 17 is decoded by an intraframe decoder 19.
And in a reverse predictive adder 21, a predictive signal applied via a switch 27 is added to the decoded signal, thus reproducing the video signal.
The reproduced video signal is outputted via a video signal output terminal 23, and is concurrently stored in a frame memory 25.
The signal is delayed by one frame in the frame memory 25, and then applied to the reverse predictive adder 21 via the switch 27. The switch 27 is controlled by a frame-synchronizing signal separated from the input data as in the case of the coding apparatus.
In the intraframe coding apparatus 5 of FIG. 1a, discrete cosine transform (DCT) is first carried out. The transformed output is then quantized, and the quantized data is variable-length encoded by such codes as Huffman codes.
In the intraframe decoders 9 and 19 of FIGS. 1 and 2, the data coded to variable lengths is first decoded to a fixed length and then transformed to a quantized representative value, which is reversely discrete cosine transformed into a reproduced signal.
Such conventional coding/decoding apparatuses process periodic independent frames. This decreases the coding efficiency because the independent frames produce a large amount of data as compared with the predictive frames.
Independent frames only are reproduced in the high-speed search in storage media. However, if the quantity of data that can be decoded by the high-speed search is the same as that at the time of ordinary regeneration, not all of the independent frames corresponding to the search speed can be decoded and the decoding apparatus comes to deliver the same frames repeatedly. This is so because the independent frames contain a larger amount of data.
Therefore, it has been devised to divide the frequency components of a video signal into a plurality of bands and decode low frequency components only.
Such a measure can reduce the amount of data to be decoded in each frame and provide smooth search pictures even in low resolution, but leaves high frequency components unused.